Add automated release workflow

[JuanVqz]

Summary

• Adds .github/workflows/release.yml triggered when a PR from a release/ branch is merged into main
• Automatically tags the commit, extracts CHANGELOG notes, creates a GitHub release, builds the gem, and pushes to RubyGems
• Automates steps 6-9 of the manual release process

Steps automated

• Tag main with the version extracted from the branch name (release/v1.x.x -> v1.x.x)
• Push tag to GitHub
• Create GitHub release with CHANGELOG notes for that version
• Build gem and push to RubyGems

Prerequisites

• Add RUBYGEMS_API_KEY as a repository secret (Settings > Secrets > Actions)

Test plan

• Verify RUBYGEMS_API_KEY secret is set in repo settings
• Open a release/vX.Y.Z PR and merge it to confirm the workflow fires end-to-end